In certain types of thermal printers, a receiver of print medium, such as paper, and a dye-donor film is moved past a print head as the print head causes an image to be transferred to the receiver. The receiver is moved past the print head in a series of repetitive passes. Each pass is made using a different color dye-donor film. In this manner, a series of overlying colored images are generated on the receiver. When the overlying images are properly registered with one another, the resultant image on the receiver is a full color image.
Registration of the overlying images is critically important to the quality of the final image. If one of the overlying images is not properly registered to the other images, then any one of a number of image artifacts occurs. One common artifact is known as a "halo effect". A halo effect of three primary colors appears around text that is printed in black when overlying images are misregistered.
Various techniques have been used in the prior art to assure accurate registration of overlying images. For example, some prior art thermal printers use clamps to positively lock a receiver on a drum. The drum is rotated to move the receiver past the print head for a first color image. The drum is then reversed and rotated to a starting position to re-align a leading edge of the receiver with the print head. The drum is then rotated again in a forward direction to move the receiver past the print head to produce a second color image. This process is repeated until a full color image is present on the receiver.
A printer which operates with a positively clamped receiver has the disadvantage of being slow to operate and requiring complex hardware. Additionally, such a printer requires a drum circumference which is equal to or larger than the length of a receiver. These are disadvantages which make such a printer undesirable for applications in typical office settings where low cost, compact size and high speed of operation are important considerations.
For typical office applications, thermal printers have been adapted to employ simpler and less expensive receiver driving systems. One such system is known as a nip driving system. A nip driving system uses a driving roller and a pinch roller to move a receiver past a print head. A receiver is driven by a nip that is formed at an interface of the pinch roller and the driving roller. As the driving roller is rotated in a forward direction, the receiver is moved past the print head to form a first color image. The driving roller is then reversed and the receiver is moved backward so that its leading edge is aligned with the print head. The driving roller is then rotated in the forward direction as a second color image is formed on the receiver. This process is repeated until all of the desired colors are printed on the receiver.
Prior art nip driven thermal printers are simpler and faster than clamping drum thermal printers, but they suffer from the disadvantage that the receiver does not always move the same distance for a given angular displacement of the driving roller. It has been found that, for example, that a forward 300 degree rotation may produce a 3.001 inch displacement of the receiver while a backward 300 degree rotation produces a 3.002 inch displacement of the receiver. We have found that these displacement variations are produced by variations in shear force that are generated between the rollers that create the nip and the receiver which is driven in the nip. A mathematical analysis of a related phenomenon is discussed in substantial detail in an article by T. C. Soong and C. Li in The Journal of Applied Mechanics, entitled "The Rolling Contact of Two Elastic-Layer-Covered Cylinders Driving a Loaded Sheet in the Nip", December 1981, Vol. 48/889.
In the prior art, these shear force variations were not recognized as factors which contributed to diminishment of image quality. There was a recognition that slippage of a receiver was a problem to be avoided, but the efforts to avoid slippage were not directed to elimination of shear force variations. Typically, prior art printers employed brute force mechanics in attempts to control receiver slippage. For example, thermal printers and plotters are disclosed in U.S. Pat. No. 4,532,525 (Takahashi), issued Jul. 30, 1985, U.S. Pat. No. 4,720,714 (Yukio), issued Jan. 19, 1988 and Japanese Patent No. 60-38181 (Amakawa), issued Feb. 27, 1985, which employ driving rollers with textured surfaces. These textured surfaces are designed to interlock with a surface of a receiver and thus avoid slippage. Another thermal printer disclosed in Japanese Patent No. 62-218165 (Oide), issued Sep. 25, 1987, uses multiple back-up rollers bearing against a receiver and a driving roller in an attempt to control slippage. Still another thermal printer is disclosed in Japanese Patent No. 61-179958 (Kataobe), issued Dec. 8, 1986, which employs a movable printing head synchronized with a paper driving system to overcome problems related to paper positioning. All of these prior art thermal printers employ complex mechanics in an effort to overcome variations in shear force which produce slippage. None of these prior art printers employ any techniques that avoid an introduction of these variation of shear forces.
In spite of these shortcomings, nip driving systems are still the driving system of choice for thermal printers intended for use in office settings. In these office applications, a color thermal printer is typically used with a personal computer as a substitute or an adjunct to a laser printer. In this context, it is very important that the color thermal printer has a low price. Because of the relative simplicity of nip driven color thermal printers, they can be manufactured at a low cost and sold at a relatively low price.
However, full acceptance of color thermal printers in office settings has not occurred in spite of the availability of inexpensive machines. This is because prior art nip-driven, color thermal printers are not capable of producing desirable images on standard or typical office paper. The typical paper used in offices today is about eight inches wide and eleven inches long. When a user of a personal computer in an office wants to make a paper output of a computer generated image, the user typically expects to use conventional office paper for the image, i.e., 8 inch wide paper. Additionally, the user expects to be able to get an image that covers substantially the entire sheet of paper. In other words, there is an expectation that any image-free borders on the paper will be relatively small.
These expectations have heretofore presented insoluble design dilemmas for producers of color thermal printers. In order to maintain decent image quality, the nip rollers of the prior-art color thermal printers were built with high mechanical strength. To be assured of low slippage, it was considered imperative that the rollers should not bend along their axes. A typical roller in a prior art, nip-driven color thermal printer has a length that is no more than three times its diameter. In such a printer, the image cannot be produced on a wide sheet of paper with a narrow image-free border. For example, it is not possible to produce an image with a one inch image-free border on typical eight inch wide office paper with a prior art nip-driven color thermal printer. Such an image requires a use of nip rollers with a radius smaller than one inch and a length about the same as the eight inch width of the paper. Such a roller would not have the requisite stiffness or resistance to bending that is required in prior art color thermal printer designs.
There are printers disclosed in the aforementioned Yukio and Kataobe patents which use rollers that appear to have a length greater than three times their diameters. However, these printers are not used to produce color images with a thermal technique, i.e., superimposed images generated on a receiver in a series of repetitive passes of the receiver across a thermal printhead. Instead the Yukio and Kataobe printers are used to produce monochromatic images with only a single pass of a receiver across a printhead.
We have found that the failure to attain highly accurate image registration in a nip driven color thermal printer is related to the nature of the prior art nip driving systems. In the prior art, the driving roller and the pinch roller are allowed to contact each other as the receiver is driven. We have found that this permits a differential shear force to develop in the receiver as the receiver is moved. These shear forces cause random variations in the surface speed of the receiver relative to the surface speed of the driving roller. Prior art printers require very rigid rollers to maintain a low rate of slippage. Thus the prior art nip driven color thermal printers have an inherent limitation on the size of an image that can be produced on a wide sheet of paper.
It is desirable therefore to provide a color thermal printer that operates at high speeds, has a low cost, and produces high resolution color image with accurate image registration. It is particularly desirable to provide such a color thermal printer which is capable of producing images with narrow image-free borders on relatively wide paper.